Several carburetors have been proposed for the purpose of creating an air-fuel mixture of substantially constant (usually stoichiometric) air-fuel ratio for an internal combustion engine. In general, it has been contemplated that such a carburetor would be used in a closed loop system having a sensor--such as a sensor that measures the oxygen content of the engine exhaust gases as an indication of the air-fuel ratio of the mixture created by the carburetor--which would initiate a feedback signal causing the carburetor to create a mixture of the desired air-fuel ratio.
Certain carburetors proposed for that application have metering apparatus which includes a solenoid armature driven between rich and lean positions according to a pulse width modulated duty cycle. The duty cycle operated solenoid thus maintains the metering apparatus in the lean position for a selected portion of the duty cycle and in the rich position for the remainder of the duty cycle, and the carburetor thus pulse width modulates the fuel flow and then averages high and low fuel flows to create a mixture of the desired air-fuel ratio.
It will be appreciated that, with such a carburetor, the solenoid must operate at a frequency sufficiently high to avoid inducing objectionable engine surge which could result from the alternate high and low fuel flows. The metering apparatus accordingly is subject to a minimum frequency limitation, and its mass and frictional characteristics must be low enough to permit its movement between the rich and lean positions at the minimum frequency over the desired range of duty cycle pulse widths.